Surgical referencing units of the kind described at the outset are described in US 2006/0173264 A1, WO 02/01148 A2 and EP 1 498 688 A1, for example. They are usually configured such that relative positions between the marker elements cannot be changed, particularly when a plurality of marker elements are provided. In other words, a geometry or marker element geometry prescribed by the marker elements of the referencing unit cannot be changed. Since the referencing units can normally be connected to different instruments or implants, before these are used a calibration of the respective instrument or implant must be carried out after connection to the referencing unit, in order to establish a definite allocation of the referencing unit to the respective instrument or implant and to transmit this information to the navigation system.
Such a procedure involves effort and is also subject to error, as an operator who does not correctly select the instrument or implant connected to the referencing unit will not obtain the desired measured values from the navigation system, namely a position and/or an orientation of the instrument or implant in space, which is usually an operating theater.
Therefore, it would be desirable to provide an improved surgical referencing unit, an improved surgical instrument and an improved surgical navigation system.